Electron beam control method, electron beam generating apparatus, apparatus using the same, and emitter

ABSTRACT

Provided is a Schottky emitter having the conical end with a radius of curvature of 2.0 μm on the emission side of an electron beam. Since a radius of curvature is 1 μm or more, a focal length of an electron gun can be longer than in a conventional practice wherein a radius of curvature is in the range of from 0.5 μm to not more than 0.6 μm. The focal length was found to be roughly proportional to the radius of the curvature. Since the angular current intensity (the beam current per unit solid angle) is proportional to square of the electron gun focal length, the former can be improved by an order of magnitude within a practicable increase in the emitter radius. A higher angular current intensity means a larger beam current available from the electron gun and the invention enables the Schottky emitters to be used in applications which require relatively high beam current of microampere regime such as microfocus X-ray tube, electron probe micro-analyzer, and electron beam lithography system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national stage of PCT International ApplicationNo. PCT/GB2006/050180, filed on Jun. 30, 2006, which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The invention relates to an electron beam control method, an electronbeam generating apparatus, a device using the same, and an emitter.

(2) Description of the Related Art

Electron guns in electron beam based instruments use two types ofcathodes (emitters); a thermionic emitter and a field emitter. Athermionic emitter uses a tungsten filament, a pointed emitter of asingle crystal or a sintered compound of lanthanium hexaboride (LaB₆) orcerium hexaboride (CeB₆). The emitter is heated and caused to emitthermal excited electrons to thereby generate an electron beam. A fieldemitter uses a sharpened conical end of an electrode on the emissionside of an electron beam and emits electrons by using a tunneling effector a Schottky effect caused by a strong electric field applied to theconical end to thereby generate an electron beam.

Note that in a case where an analysis or observation is carried out in asmall region, an electron beam with a high brightness is required inorder to reduce a diameter thereof (here, the “brightness” is defined asthe current density per unit solid angle of the electron beam).Therefore, in recent years, a field emitter has been adopted, instead ofa thermionic emitter that has been conventionally employed, in ascanning electron microscope (hereinafter also referred to as “SEM” forshort) and an electron probe microanalyzer (hereinafter also referred toas “EPMA” for short) as well as other electron beam based instruments;transmission microscopy, electron beam lithography, inspection tools,etc. in analysis or observation in a small region to thereby improve aspatial resolution.

There are two types of field emitters, a cold field emitter and athermal field emitter. In the case of a cold field emitter, the conicalend of an emitter is normally made from a single crystal fine tungstenwire and is subjected to a strong electric field at room temperaturewhereby electrons, in the single crystal, are emitted using a tunnelingeffect, so that an electron beam is generated. In the case of a thermalfield emitter, the conical end of an emitter made from a single crystalfine tungsten wire coated with zirconium oxide (ZrO) is heated whilebeing subjected to a strong electric field which causes electrons to beemitted using a Schottky effect, so that an electron beam is generated.Since a thermionic emitter uses a Schottky effect as described above, itis also called a Schottky emitter.

In a Schottky emitter, a zirconium oxide layer coating the conical endof the emitter has an effect of reducing a work function of a crystalface, formed in the conical end, and which is a (100) crystal plane.Therefore, a uniform, strong electron beam is emitted and extracted fromthe conical end. Note that a Schottky emitter technology is disclosed inU.S. Pat. Nos. 145,042 and 145,043.

In the case of a field emitter, as described above, the current densityis, however, higher than that of a thermionic emitter. In the case of afield emitter, the electron source diameter, where an electron beam isemitted from in an electron gun configuration, is very small, as shownin FIG. 9B, in comparison with a thermionic emitter of FIG. 9A (FIG. 9Bshows a Schottky emitter). An electron source diameter is several tensof μm in a case of a thermionic emitter, while in a field emitterrepresented by a Schottky emitter, an electron source diameter isseveral tens of nm. If an electron source area of a thermionic emitteris indicated by dS_(TE), and an electron source area of a field emitteris indicated by dS_(FE), the areas are different from each other by upto six orders of magnitude.

On the other hand, if a solid angle of an electron beam is indicated bydΩ and a beam current value (current value is indicated by I_(b)), asolid angle dΩ of the electron beam varies according to a beam currentvalue I_(b) to be required. If an axial brightness of the electron beamis indicated by B, a beam current value I_(b) is given by the followingequation (1) with an electron source area dS and a solid angle dΩ.I _(b) =B×(dS×dΩ)  (1)

In a case where a larger beam current is necessary, it is understoodfrom equation (1) that an effective solid angle dΩ increases for fixedbrightness and source area.

A Schottky emitter is much higher in brightness than a thermionicemitter (by about three orders of magnitude). However, since an electronsource area dS_(FE) is smaller than dS_(TE) by up to six orders ofmagnitude, a solid angle dΩ of an electron beam in a case where the samebeam current is secured is larger in a Schottky emitter than that in athermionic emitter. That is, if a solid angle of an electron beam in athermionic emitter is indicated by dΩ_(TE) and a solid angle of anelectron beam in a field emitter represented by a Schottky emitter isindicated by dΩ_(FE), a relation expressed by the following equation (2)is established.dΩ _(FE) >dΩ _(TE)  (2)

That is, an angular current density which is the current per unit solidangle for a Schottky emitter is smaller than that of a thermionicemitter although the Schottky emitter has a higher axial brightness thanthe thermionic emitter.

Since, with a larger solid angle, an electron beam is diverged,collimation is required. As a result, in a field emitter, an aberrationin an accelerating and condenser lens section downstream from theemission side exerts a large influence, so that a characteristic of theemitter, which would be by nature high in brightness, is degraded by aninfluence of the aberration, and the “apparent brightness” decreases asa beam current increases. FIG. 10 is a graph showing relationshipsbetween a beam current value and brightness in the case where a Schottkyemitter is employed as an example of a field emitter and the case wherea tungsten filament emitter is employed as an example of a thermionicemitter. The abscissa is assigned to a beam current and the ordinate isused for plotting brightness. A dotted line is a curve concerning atungsten filament emitter and a solid line shows a curve concerning aSchottky emitter. Note that in the Schottky emitter, the curve wasobtained under the conditions where the emission current density j_(s)is 1.0×10⁴ A/cm², an emitter temperature T is 1800 K and an angularcurrent density J_(ΩSE) is 0.429 mA/str, while for the thermionicemitter, the curve was obtained under the conditions where the emissioncurrent density j_(s) is 3 A/cm², an emitter temperature T is 2800 K andan angular current density J_(ΩW)=140 mA/str. The term “W filament”indicates the tungsten filament operated in the thermionic mode and theterm “SE” indicates the Schottky emitter.

In a case of a thermionic emitter represented by a tungsten filament,the angular current density is high; therefore a decrease in brightnessis not problematical in a practical aspect giving a reduced brightnesswhen the current is in the neighborhood of a value in the range of 10 μAto 20 μA. On the other hand, in a case of a field emitter represented bya Schottky emitter, an angular current density is lower and an electronsource diameter is smaller; therefore, the brightness begins to decreasewhen the beam current is in the neighborhood of 1 nA and the brightnessdecreases by 6 orders of magnitude at a beam current 1 μA.

Since a beam current employed in a case of a scanning electronmicroscope (SEM) is at a level of nA or less, no reduction in brightnessis observed with a Schottky emitter in a case where the emitter is usedin SEM. Thus, a Schottky emitter can be used in SEM. However, in a caseof a device where a beam current at a level of sub μA or μA is requiredas in a electron probe micro analyzer (EPMA), reduction in thebrightness is observed at a level of sub μA or μA for a Schottkyemitter; therefore, even if a Schottky emitter is employed ininstruments such as EPMA, only an electron beam with a low brightnesscan be used. Hence, it is impossible to employ a field emitter ininstruments such as EPMA in a practical sense.

SUMMARY OF THE INVENTION

The invention has been made in light of such circumstances and it is anobject of the invention to provide an electron beam control method, anelectron beam generating apparatus, capable of freely setting theangular current density in a Schottky emitter.

The following findings and knowledge have been obtained in order toachieve such an objective.

As shown in FIG. 11A, a Schottky emitter 201 has a construction in whicha conical end 201 a of an emitter 201 on an emission side B of anelectron beam is, as described above, sharpened in the shape of a cone.Note that FIG. 11B is a schematic diagram in which the conical end 201 athereof is enlarged and if a radius of curvature of the conical end 201a is indicated by R, R is in the range of R<0.5 μm to 0.6 μm.

Note that if trajectories of electron beams emitted from an electron gun(emitter) are called as “cathode trajectories”, a primary characteristicof the cathode trajectories is characterized by electron gun focallength. (S. Fujita and H. Shimoyama, J. Electron Microscopy, 54(4),331-343 (2005)) FIG. 12 is a diagram schematically showing the emitter(cathode) of an electron gun. If an angle is formed between an electrontrajectory emitted normal to (at a given angle relative to) the cathodesurface at position ξ and an optical axis on a reference place (a driftregion) is indicated by β as shown in FIG. 12, a focal length f isdefined by the following differential equation (3).1/f=−(∂ sin β/δξ)|_(u=0ξ→0)  (3)

It is seen in the equation (3) that the reciprocal of the electron gunfocal length is the limiting ratio of the sine of the emerging ray angleto the off-axis distance of the starting position for the electronsemitted perpendicularly to the cathode surface. A crossover diameter,the minimum beam diameter of an electron beam formed along the opticalaxis (an electron source diameter) and an angular current density areobtained from the focal length f defined by the equation (3).

If an electron source diameter is indicated by d_(co), a Boltzmann'sconstant k, an absolute temperature T, the electronic charge e, apotential (an extraction potential) at an extractor electrode V_(ext)and a current density at a cathode (a cathode current density) j_(s),then an electron source diameter d_(co) and an angular current densityJ_(Ω) are given by the following equations (4) and (5), respectively.d _(co)=2×f×{(k×T)/(e×V _(ext))}^(1/2)  (4)J _(Ω) =f ² ×j _(s)  (5)

If a focal length f is longer, an electron source diameter d_(co) islarger as is understood from the equation (4) and an angular currentdensity J_(Ω) is also raised as is understood from the equation (5).Hence, in order to set an angular current density J_(Ω) in a Schottkyemitter with reasonable freedom, it is only required to adjust a focallength f.

Note that the increase (or decrease) of the angular current densityJ_(Ω) by the change in the electron gun focal length necessarilyaccompanies the increase (or decrease) of the electron source diameterd_(co). Consequently the brightness B itself is independent of the focallength as is shown below,B=J _(Ω)/π(d _(co)/2)²=(1/π)(e j _(s) /kT)V _(ext),  (6)Hence, in order to control both of the electron optically importantparameters, i.e. the brightness B and the angular current density J_(Ω),it is necessary to have simultaneously under control the electron gunfocal length f and the cathode current density j_(s).

Let's start with the electron gun focal length. Then, attention will bepaid to the equation (3), which defines a focal length f. It was foundin this invention that the electron gun focal length can be adjusted byaltering the shape of the Schottky emitter. By scaling up or down theemitter tip radius R it is possible to increase or decrease the off-axisdistance ξ which corresponds to a fixed emerging ray angle β. A focallength f defined by the equation (3) is obtained by fitting v (=sin β)in FIG. 12 using the following equation (6).v=(−1/f)×ξ+ε×ξ³  (7)

FIG. 13 is a graph showing relationships between ξ and v in cases wheretwo Schottky emitters with different radii of curvature R were employed.The abscissa is assigned to ξ and, also, the ordinate is used forplotting v. To be concrete, employed here is a Schottky emitter formedwith a radius of curvature R of a conventional dimension of 0.6 μm and aSchottky emitter formed with a radius of curvature of 2.0 μm, which islarger than conventional size cathodes. As shown in FIG. 13, a curve ofa conventionally standard Schottky emitter (R=0.6 μm) is marked with theterm “standard SE”, while a curve of a Schottky emitter (R=2.0 μm)having a radius of curvature R larger than the conventional size ismarked with the term “Giant SE”.

In FIG. 13, inclinations of both curves in the vicinity of ξ=0 take avalue (−1/f). By comparison in inclination, the Giant SE with a smallerinclination has a focal length f longer than the standard SE with alarger inclination.

As described above, findings and knowledge have been obtained that byadjusting the radius of curvature of the conical end of an emitter onecan control a focal length and consequently a angular current densitycan be freely set. In particular, findings and knowledge have beenobtained that if a radius of curvature of the conical end of an emitteris adjusted to be larger than those employed for conventional Schottkyemitters, the focal length becomes longer and consequently the angularcurrent density can be increased.

Next, we shall investigate the cathode current density. With Schottkyemitters and cold field emitters the current density j_(s) is a functionof the electric field strength at the cathode. Since in the case of thepoint cathode tip the electric field is enhanced by its small curvatureradius, the change in the radius usually influences the field strength.Larger emitters would have a lower field if the other electrodeconfigurations and the applied voltage were unchanged. Because theapplied voltages cannot be augmented indefinitely without risk of thedischarge, some compensation in the electrodes configuration isnecessary in order to recover a high enough electric field with largerradius emitters. One effective way of its realization is to make theprotrusion length of the emitter from the suppressor longer in theSchottky emitter module configuration which consists of an emitter, asuppressor and an extractor. By setting an appropriate protrusion lengthin accordance with the emitter tip radius it is possible to secure highenough tip field under reasonable applied voltage condition.

Therefore, the invention reported here based on the findings andknowledge obtained by the inventors has the following configuration.

That is, an electron beam control method related to the invention is anelectron beam control method of generating an electron beam from anelectron emitter by Schottky effect by applying an electric field tosaid emitter, wherein said electron emitter comprises a sharp tip ofconical shape and the method comprises the step of adjusting a radius ofcurvature of said tip, thereby to control the focal length of anelectron beam emitted from said tip and thereby control the angularcurrent density of said beam.

A preferable example of the electron beam control method related to theinvention is to select a radius of curvature in the range of 1 μm ormore. By selecting a radius of curvature in the range of 1 μm or more, afocal length is controlled so as to be longer than in a conventionalcase where a radius of curvature is in the range from 0.5 μm to no morethan 0.6 μm and, besides, a angular current density can be controlled toa higher value than in the conventional case.

Preferably, the electron beam control method related to the inventioncomprises:

a protrusion length adjusting step of adjusting a protrusion length thatis a length of the conical end from a suppressor electrode which islocated on the side opposite to the emission direction among the twoelectrodes applying an electric field and to which a negative voltage isapplied; and

a combined range setting step of setting a combined range of theprotrusion length and the radius of curvature based on the value of theelectric field, wherein

in the protrusion length adjusting step, a protrusion length is selectedin the combined range at a radius of curvature adjusted in the curvatureradius adjusting step.

According to the electron beam control method of selecting a protrusionlength, a protrusion length is adjusted to thereby enable an electricfield strength at the conical end to be controlled. Hence, in a Schottkyemitter, a protrusion length is adjusted so as to enable a necessaryelectric field to be secured. A characteristic of an electric field withthe protrusion length also changes according to an emitter's radius ofcurvature. Therefore, a combined range of a protrusion length and aradius of curvature are set in the combined range setting step based ona value of an electric field. A protrusion length is selected at aradius of curvature adjusted in the curvature radius step within thecombined range. The selection enables a protrusion length to be adaptedfor a radius of curvature.

A preferable example of the electron beam control method selecting aprotrusion length is that the radius of curvature is selected in therange of 1 μm or more and 4 μm or less in the curvature radius adjustingstep, and a protrusion length is selected in the range of 200 μm or moreand 1500 μm or less from the combined range at the radius of curvatureadjusted in the curvature radius adjusting step (see FIG. 4), therebyenabling a controlled increase of the angular current density to higherthan conventional values to be realized while maintaining the high beambrightness of the Schottky emitter by ensuring the high cathode currentdensity j_(s) with the appropriate electric field at the tip.

Preferably, the electron beam control method related to the inventioncomprises: an emitter forming step of adjusting the protrusion length ofthe conical end and, also, forming the emitter to avoid revealing a(100) crystal plane on the lateral surface of the emitter on theemission side outward from the suppressor electrode that is applied witha negative voltage and located on the side opposite the emission sideamong the two electrodes applying the electric field.

According to the electron beam control method applying forming not toreveal a (100) crystal plane in the side surface portion of an emitteron the emission side outward from the suppressor electrode, anunnecessary (100) plane is hidden in the rear part on the side oppositethe emission side of the suppressor electrode, which enables anunnecessary extraction current to be suppressed. The term “emitter sidesurface” means a surface parallel to an emission direction of theelectron beam.

An electron beam generating apparatus related to the invention is anelectron beam generating apparatus comprising: an emitter having theconical end sharpened in the shape of a cone on the emission side of anelectron beam; and two electrodes applying an electric field to theconical end of the emitter, wherein the electric field is applied to theconical end to thereby emit electrons using a Schottky effect, so thatan electron beam is generated,

the electron beam generating apparatus having an improvement that aradius of curvature of the conical end is 1 μm or more.

the electron beam generating apparatus having an improvement that theaxial distance of the emitter tip from the suppressor electrode (theprotrusion length) is selected from the combined range of the radius ofcurvature and the axial distance of the emitter tip from the suppressorelectrode, where range of values affords a desired electric field inwhich said emitter is located.

According to the electron beam generating apparatus related to theinvention, since a radius of curvature of the conical end is 1 μm ormore, a focal length can be longer as compared with a conventional casewhere a radius of curvature is in the range of 0.5 μm and not more than0.6 μm and, furthermore, the obtained angular current density can behigher than values obtained under conventional set-ups. In addition,since the emitter tip field is kept high enough by selecting anappropriate protrusion length in accordance with the tip radius, thecathode current density and the beam brightness can be maintained.

An electron beam device comprising the electron beam generatingapparatus related to the invention is a device using electron beamgenerating apparatus including: an emitter having the conical geometryend sharpened in the shape of a cone on the emission side of an electronbeam; and two electrodes applying an electric field to the conical endof the emitter, wherein the electric field is applied to the conical endto thereby emit electrons using a Schottky effect, so that an electronbeam is generated, the electron beam generating apparatus having animprovement that

a radius of curvature of the conical end is 1 μm or more,

the device further comprising:

processing means conducting a predetermined processing based on anelectron beam generated by the electron beam generating apparatus.

According to the device using electron beam generating apparatus relatedto the invention, since a radius of curvature of the conical end is 1 μmor more, a focal length can be longer than a conventional case where aradius of curvature is in the range of 0.5 μm and not more than 0.6 μmand an angular current density can be higher than conventional. Inaddition, since an angular current density is higher than conventional,the brightness hardly decreases with beam current, so that theprocessing means can conduct a predetermined processing using anelectron beam with a high brightness, thereby enabling the processingmeans to be applied to various devices.

An example of devices using electron beam generating apparatus relatedto the invention is an electron probe microanalyzer conducting ananalysis or observation of a specimen, and the processing means conductsan analysis or observation of a specimen by irradiating the specimenwith an electron beam to obtain an X-ray image based on X-rays generatedfrom the specimen, or by irradiating a specimen with an electron beam toobtain an electron beam image based on secondary electrons or reflectedelectrons generated from the specimen.

An electron probe microanalyzer is optimal for analyzing or observing asmall region of a specimen.

Another example of the device using electron beam generating apparatusrelated to the invention is an X-ray tube, and the processing means is atarget generating X-rays by collision with an electron beam.

Since the X-ray tube is equipped with an emitter emitting an electronbeam with a high brightness, an angle of an electron beam when a targetis irradiated therewith can be suppressed to be small, thereby enablingan X-ray generating area on the target to be small. Therefore, a spatialresolution of an X-ray image can be improved.

Still another example of the device using electron beam generatingapparatus related to the invention is an electron beam lithographysystem, and the processing means conducts lithography using an electronbeam.

Since an electron beam lithography system is equipped with an emitteremitting an electron beam with a high brightness, an angle of anelectron beam converging at one point on a pattern used in lithographycan be suppressed to be small, thereby, enabling a spatial resolution ofa pattern for lithography formed on a target of lithography to bebetter.

An example of the electron beam generating apparatus or the device usingelectron beam generating apparatus of the invention is as follows: asuppressor electrode and the emitter are disposed so that a protrusionlength, that is a length to the topmost point of the conical end fromthe suppressor electrode, is in the range of 200 μm or more and 1500 μmor less, and a radius of curvature of the conical end is in the range of1 μm or more and 4 μm or less. Since a radius of curvature is in therange of 1 μm or more and 4 μm or less and a protrusion length is in therange of 200 μm or more and 1500 μm or less, an angular current densitycan be higher than in a conventional practice and, at the same time, anelectric field at the conical end can be controlled.

In a electron beam generating apparatus or a device using electron beamgenerating apparatus, the emitter preferably has a form not to reveal a(100) crystal plane in the emitter side surface portion on the emissionside outward from a suppressor electrode that is located on the sideopposite the emission side among the two electrodes applying an electricfield, and applied with a negative voltage.

In this case, since unnecessary (100) planes are hidden in the rear parton the side opposite the emission side of the suppressor electrode, anunnecessary extraction current is suppressed.

An emitter related to the invention is an emitter generating an electronbeam in which the conical end of an emitter on the emission side of anelectron beam is sharpened in the shape of a cone and is applied with anelectric field to thereby emit electrons using a Schottky effect, theemitter having an improvement that

a radius of curvature of the conical end is 1 μm or more.

According to an emitter related to the invention, since a radius ofcurvature of the conical end is 1 μm or more, a focal length can belonger than a conventional practice where a radius of curvature is inthe range of from 0.5 μm to not more than 0.6 μm and an angular currentdensity can be higher than conventional.

In a case where a protrusion length of an emitter is adjusted in therange of 200 μm or more and 1500 μm or less with a radius of curvaturein the range of 1 μm or more and 4 μm or less according to the proposedcombined range of radius curvature and the protrusion length whichensures high enough tip electric field (see FIG. 4), the beam brightnesscan be kept high.

An emitter related to the invention preferably has a form not to reveala (100) crystal plane in the emitter side surface portion.

In this case, unnecessary (100) crystal planes are hidden in the rearpart on the side opposite the emission side of the suppressor electrode,which enables an unnecessary extraction current to be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there are shown in thedrawings several forms of which are presently preferred, it beingunderstood, however, that the invention is not limited to the precisearrangement and instrumentation shown.

FIG. 1A is a schematic diagram showing a Schottky emitter related to oneexample of the invention;

FIG. 1B is an enlarged schematic diagram of the conical end of theemitter;

FIG. 1C is an enlarged diagram for comparison with the conical end of aconventional emitter;

FIG. 2 is a schematic diagram of an electron beam generating apparatusequipped with the Schottky emitter;

FIG. 3 is a schematic block diagram of an electron probe microanalyzer(EPMA) equipped with the electron beam generating apparatus;

FIG. 4 is a graph roughly showing a combined range of a protrusionlength and a radius of curvature;

FIG. 5A is a schematic diagram showing a configuration of a suppressorelectrode and a Schottky emitter processed by means of a DC etchingmethod or a similar means to produce similar shapes;

FIG. 5B is a schematic diagram showing a configuration of a suppressorelectrode and a Schottky emitter processed by means of an AC etchingmethod or other means to produce similar shapes;

FIG. 6 is a graph showing relationships between a beam current value anda reduced brightness in an example of the invention, a standard Schottkyemitter of a conventional technology, and a tungsten filament emitter;

FIG. 7 is a schematic block diagram of a microfocus X-ray tube equippedwith a Schottky emitter;

FIG. 8 is a schematic block diagram of an electron beam exposure systemequipped with a Schottky emitter;

FIG. 9A is a diagram schematically showing an electron sourcecharacteristics when an electron beam is emitted from an electron gun ofa thermionic emitter;

FIG. 9B is a diagram schematically showing an electron sourcecharacteristics when an electron beam is emitted from an electron gun ofa field emitter;

FIG. 10 is a graph showing relationships between a beam current valueand brightness in a conventional Schottky emitter and a tungstenfilament emitter;

FIG. 11A is a schematic diagram showing a conventional Schottky emitter;

FIG. 11B is an enlarged schematic diagram of the conical end of theemitter;

FIG. 12 is a diagram schematically showing an emitter of an electron gunand the definition of the electron gun focal length; and

FIG. 13 is a graph describing the findings and knowledge leading to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Detailed description will be given of a preferred embodiment of theinvention below based on the accompanying drawings.

FIG. 1A is a schematic diagram showing a Schottky emitter related to oneexample of the invention. FIG. 1B is an enlarged schematic diagram ofthe conical end of the emitter. FIG. 1C is an enlarged diagram forcomparison with the conical end of a conventional emitter. FIG. 2 is aschematic diagram of an electron beam generating apparatus equipped withthe Schottky emitter. FIG. 3 is a schematic block diagram of an electronprobe microanalyzer (EPMA) equipped with the electron beam generatingapparatus.

A Schottky emitter 1 related to the example, as shown in FIG. 1A, hasthe conical end 1 a sharpened in the shape of a cone on the emissionside of an electron beam B (here, the symbol “B” in the figure denotesthe electron beam and should not be confused with the quantityrepresenting the brightness). The Schottky emitter 1 has a constructionin which a zirconium oxide layer is coated on a single crystal wire oftungsten. As shown in FIG. 1B, a radius of curvature R of the conicalend 1 a is 2.0 μm and preferably in the range of 1 μm or more, which islarger as compared with a conventional conical end 201 a in the range of0.5 μm and not more than 0.6 μm (see FIG. 11B and FIG. 1C). Note that inFIG. 1C, the conical end 1 a of a Schottky emitter 1 related to theexample is shown with a two-dot chain line.

An electron beam generating apparatus 10 equipped with the Schottkyemitter 1, as shown in FIG. 2, includes two electrodes 2 and 3 applyingan electric field to the conical end 1 a of the Schottky emitter 1; ananode 4 extracting an electron beam B; and a condenser lens 5 convergingthe electron beam B. A portion consisting of the Schottky emitter 1 andthe electrodes 2 and 3 (a suppressor electrode 2 and an extractorelectrode 3 described later) is called an electron source. The electronsource is easy to be understood with a potential of the Schottky emitter1 as a reference (in FIG. 2, the potential is 0 V. In an actual case, apotential of the emitter 1 is usually at a negatively high potential).The conical end 1 a is heated under a strong electric field appliedthereto by the electrodes 2 and 3 to thereby emit electrons using aSchottky effect and the electron beam B is thus generated by theelectron beam generating apparatus 10. The electron beam generatingapparatus 10 corresponds to an electron beam generating apparatus of theinvention and also corresponds to an electron beam generating apparatus.

Of the two electrodes 2 and 3, the electrode 2 located on the sideopposite the emission side and applied with a negative voltage (in FIG.2, −300 V) is a suppressor electrode 2 and the electrode 3 located onthe emission side and applied with a positive voltage (in FIG. 2, 6423V) is an extractor electrode 3.

The anode 4 is disposed opposite the Schottky emitter 1 serving as acathode and applied with a positive voltage with respect to the emitter1. The anode 4 attracts the electron beam B emitted from the Schottkyemitter 1. The electron beam B is accelerated by attraction by the anode4.

The condenser lens 5 is constructed in the shape of a ring. A current issupplied into the condenser lens 5 from a lens power supply not shown tothereby generate a magnetic field to converge the electron beam B in asimilar way to light in an optical condenser lens.

Description will be given of a configuration of the Schottky emitter 1,the electrodes 2 and 3, the anode 4 and the condenser lens 5 in theelectron beam generating apparatus 10, again, with reference to FIG. 2.The suppressor electrode 2 and the extractor electrode 3 are disposedwith a spacing of 700 μm therebetween. Even though the spacing issimilar to the conventional Schottky emitter configurations, thedisposition of each electrode is unique. The length from the suppressorelectrode 2 to the topmost point of the conical end 1 a is indicated byL_(st). On the other hand, a length to the extractor electrode 3 fromthe topmost point of the conical end is indicated by L_(TE). Hence, arelation that L_(st)+L_(TE)=700 μm is established. If the emitter isoperated with the same protrusion length L_(ST) of 250 μm asconventional, an electric field strength F at the conical end 1 a cannotbe secured to be a necessary value (in this case, F=1×10⁹ V/m).Therefore, a protrusion length L_(ST) is set to be longer thanconventional case in order to raise an electric field strength F to anecessary value (1×10⁹ V/m). In a case of a Schottky emitter where aradius of curvature R of the conical end 1 a is 2.0 μm, the Schottkyemitter 1 and the suppressor electrode 2 are disposed so that aprotrusion length L_(ST) is 400 μm. Therefore, L_(TE) is 300 μm (=700μm−L_(ST)).

In order to secure a necessary electric field strength F at the conicalend 1 a, a protrusion length L_(ST) is adjusted so as to be adapted fora radius of curvature R. That is, a characteristic of an electric fieldversus a protrusion length L_(ST) also varies depending on a radius ofcurvature R. Hence, as shown in FIG. 4, a combined range of a protrusionlength L_(ST) and a radius of curvature R are set in advance based on avalue of an electric field necessary for field emission in the Schottkymode. In this case, a protrusion length L_(ST) and a radius of curvatureR are individually altered to estimate the combined range suitable forthe necessary electric field strength F (1×10⁹ V/m) (see a crosshatchedportion in FIG. 4). That is, a combined range of a radius of curvature Rand a protrusion length L_(ST) necessary for an operation of theSchottky emitter 1 is defined by the hatched area in FIG. 4. Note thatin FIG. 4, there is shown a combination of a radius of curvature R and aprotrusion length L_(ST) (where R=0.5 μm and LST=250 μm) of aconventional standard Schottky emitter (with R=0.5 μm) with a mark “x”.

A distance between the anode 4 and the condenser lens 5 is indicated byL. In a case of a thermionic emitter, the anode 4 and the condenser lens5 are spaced with a distance of a value of the order of L=100 mm. ThoughL is longer and a lens aberration coefficient is larger, the problem oflarger aberrations leading to larger beam diameter is not incurred sincea thermionic emitter has a large angular current density. Contrastthereto, in a case of a Schottky emitter, since an angular currentdensity is smaller, the intrinsically high brightness of the Schottkyemitter is degraded by an increase in lens aberration coefficients.Hence, in a case of a Schottky emitter, it is preferable that in orderto suppress a lens aberration coefficient, L is set to be as close to 0mm as possible to thereby locate the condenser lens 5 so as to be closerto the side of the Schottky emitter 1.

If a Schottky emitter 1 is processed using a direct current (DC) etchingmethod or other suitable means, unnecessary (100) crystal planes arerevealed, as shown in FIG. 5A, forward from the suppressor electrode 2,that is on the emission side (see hatching with oblique lines inclinedto the right in the figure). A work function of a (100) crystal planedecreases by the action of a zirconium oxide layer and an unnecessaryextraction current is extracted with a result of increasing a load on apower supply. As a result, larger outgas rate, which is a gas load fromthe surrounding electrode surfaces, is generated to degrade the degreeof vacuum in the vicinity of the emitter. The term, a DC etching method,is an etching conducted without altering polarities of electrodes usedin the etching.

Contrast thereto, in a case where an alternate current (AC), or similarmeans are used in forming a Schottky emitter 1, the etching can beconducted, as shown in FIG. 5B, so that the conical end 1 a in the shapeof a cone is longer with oblique lines in profile. The AC etching methodenables not only an etched macro face with oblique lines in profile tobe obtained, but also a crystal face different from a (100) to bemicroscopically produced. Hence, by processing the Schottky emitter 1with an AC etching method, a (100) is not revealed on a crystal surfacein the emitter side surface portion on the emission side outward fromthe suppressor electrode 2. The term “emitter side surface” is a surfaceparallel to the emission direction of the electron beam B. Therefore,hatching with oblique lines inclined to the right in the figureindicates (100) crystal planes in the emitter side surface portion. Withsuch a construction adopted, unnecessary (100) crystal planes are hiddenin the rear part on the side opposite the emission side of thesuppressor electrode 2, which enables an unnecessary extraction currentto be suppressed. The term, an AC etching method, is to conduct etchingwhile polarities of electrodes used for the etching are alternated.

Then, description will be given of a method for controlling an electronbeam. To begin with, a radius of curvature R of the conical end 1 a isadjusted. A radius of curvature R is adjusted to a value larger thanconventional in order to control the electron gun focal length f to belonger and to control the angular current density to be higher. Since aradius of curvature of a conventional conical end 1 a is in the range of0.5 μm to no more than 0.6 μm, it is preferable to select a radius ofcurvature R in the range of 1 μm or more. In one embodiment given hereby way of example, a radius of curvature R is selected to be 2 μm.

As described above, a combined range of a protrusion length L_(ST) and aradius of curvature R is set in advance based on an electric fieldvalue. The suppressor electrode 2 and the Schottky emitter 1 aredisposed by determining a protrusion length L_(ST), which is a lengthfrom the suppressor electrode 2 to the topmost point of the conical endof the Schottky emitter 1. In adjustment of a protrusion length L_(ST),a protrusion length L_(ST) is selected from the combined range shown inFIG. 4 at an adjusted radius of curvature R.

In FIG. 4, a desirable combined range of a radius of curvature R and theprotrusion length L_(ST) is defined in the range selected in the range 1μm<R<4 μm and 200 μm<L_(ST)<1500 μm. In one embodiment shown here by wayof example, a combination of a radius of curvature R and a protrusionlength L_(ST) (R=2.0 μm and L_(ST)=400 μm) is selected. By selecting acombination point in the combined range in FIG. 4, an electric fieldstrength F at the conical end 1 a can be controlled (in the example,F=1×10⁹ V/m).

A focal length f or the electron beam B is controlled by a radius ofcurvature R thus adjusted. An angular current density of the electronbeam B is controlled by a controlled focal length f, while the beambrightness is maintained at its intrinsically high value by guaranteeinga large enough tip electric field through the emitter protrusion lengthadjustment.

In a case where a radius of curvature of the conical end 1 a is set inthe range of 1 μm or more and 4 μm or less and a protrusion lengthL_(ST) of the Schottky emitter 1 is adjusted in the range of 200 μm ormore and 1500 μm or less, an angular current density can be higher thanconventional and at the same time, an electric field F at the conicalend 1 a can be controlled to maintain the high beam brightness.

Since an angular current density is higher than conventional geometry, abrightness is scarcely reduced even at relatively high beam currents andin the EPMA 50 of the example, the elementary analysis processingsection 20 and the surface observation processing section 30 can conductpredetermined processing such as elementary analysis processing andsurface observation processing, respectively with a high brightnesselectron beam. Therefore, the invention can be applied to variousapparatuses represented by such an EPMA 50.

Note that in a case where an electron beam generating apparatus 10related to the example is used in EPMA 50, the following effect isexerted. That is, EPMA 50 requires a beam current at a level of sub μAor μA, and it is also confirmed in FIG. 6 that in EPMA 50, no reductionin the brightness in the Schottky emitter 1 is observed even with alevel of sub μA or μA.

FIG. 6 is a graph showing relationships between a beam current value anda brightness in a Schottky emitter 1 (R=2.0 μm) related to the example,a standard Schottky emitter (R=0.6 μm) of a conventional technology anda tungsten filament emitter as a thermionic emitter. That is, the graphof FIG. 6 is obtained by adding the graph showing a relationship betweena beam current value and a brightness in the Schottky emitter 1 relatedto the example to FIG. 10. FIG. 6 was obtained in the same condition asin FIG. 10. In the Schottky emitter 1 related to the example, however,the relationships were obtained in the conditions that a current densityj_(s) is 1.0×10⁴ A/cm², a temperature T is 1800 K and a angular currentdensity J_(ΩGSE) is 2.22 mA/str. A curve drawn with a dotted line is acurve of the tungsten filament emitter and two curves drawn with a solidline in FIG. 6 is a curve of the Schottky emitter, wherein “Giant SE” inthe graph indicates the curve of a Schottky emitter 1 (R=2.0 μm) relatedto the example having a radius of curvature R of the conical end 1 alarger than conventional and “Standard SE” indicates the curve of aconventional standard Schottky emitter (R=0.6 μm). The mark of “Wfilament” in the graph indicates a tungsten filament.

It is found from FIG. 6 that in the case of a conventional standardSchottky emitter, an angular current density is low and an electronsource diameter is small; therefore, a brightness begins to decrease ata beam current in the vicinity of 1 nA or greater and reduces by as muchas 6 orders of magnitude at a level of 1 μA. Contrast thereto, in a caseof a Schottky emitter 1 related to the example, an angular currentdensity is high; therefore, it has been confirmed that a brightness ishard to decrease as compared to a standard Schottky emitter and abrightness decrease starts at about 1 μA if a position of the condenserlens 5 is properly selected. Hence, a Schottky emitter can be applied toa device requiring a beam current at a level of sub μA or μA such as inan EPMA 50.

The invention can be modified in the following way without limiting tothe embodiment.

(1) In the example, description was given of an electron probemicroanalyzer (EPMA) as an example of a device using an electron beamgenerating apparatus, no specific limitation is imposed on a device asfar as an electron beam generating apparatus is used therein. Forexample, the device may be a scanning electron microscope (SEM), atransmission electron microscope (hereinafter also referred to as “TEM”for short), a microfocus X-ray tube, an Auger electron spectrometer, anelectron beam lithography system and an electron beam writer. Atransmission electron microscope (TEM) can observe a projected image bycausing an electron beam to be transmitted through a thin film specimenwith a thickness of the order in the range of several tens to hundredsof nanometers. A microfocus X-ray tube generates an X-ray beam with asmall diameter of the order in the range of from sub μm to several μm bycausing an electron beam to collide with a target. An Auger electronspectrometer examines energy of Auger electrons to conduct an elementaryanalysis on a specimen. An electron beam lithography system conductslithography with an electron beam instead of light in a conventionaltechnology. An electron beam writer produces “masters” for high densityoptical disks.

Description will be given not only of the microfocus X-ray tube but alsoof the electron beam exposure system as an example of electron beamlithography system. FIG. 7 is a schematic block diagram of a microfocusX-ray tube and FIG. 8 is a schematic block diagram of an electron beamexposure system.

A microfocus X-ray tube 70 equipped with an electron beam generatingapparatus 10, as shown in FIG. 7, includes a target 60 generating X-raysby collision of an electron beam therewith. The electron beam generatingapparatus 10 is equipped with not only the suppressor electrode 2, theextractor electrode 3, the anode 4 and the condenser lens 5, but also aniris lens 6 and an objective lens 7. The iris lens 6 has an aperture 6 ahaving a diameter reducing hole that defines converging angle of anelectron beam B. The condenser lens 5, the iris lens 6, the objectivelens 7 and the target 60 are sequentially disposed in order from theupstream side (the emitter 1 side) to the downstream side in anirradiation direction of the electron beam B. The target 60 is formedfrom a material generating X-rays represented by tungsten. The target 60corresponds to processing means of the invention.

Since the microfocus X-ray tube 70 has a Schottky emitter 1 in which anelectron beam brightness does not deteriorate at a high beam currentcondition, an angle of the electron beam B when the target 70 isirradiated with the electron beam B can be suppressed to be small,thereby enabling the electron beam size focused on the target to besmall. Consequently, an X-ray generating region on the target 60 can besmaller, and a spatial resolution of an X-ray image is improved.

The electron beam exposure system 90 equipped with the electron beamgenerating apparatus 10 includes an exposure processing section 80conducting exposure on a substrate W as shown in FIG. 8. The electronbeam generating apparatus 10 is equipped with: the suppressor electrode2; the extractor electrode 3; the anode 4; and the condenser lens 5. Theexposure processing section 80 is equipped with: irradiation lens 81,image forming lens 82; a shaping aperture 83; a blanker 84; a reticle85; and a contrast aperture 86. The reticle 85 is an original drawing ofan exposure pattern. The exposure processing section 80 corresponds toprocessing means of the invention.

In the electron beam exposure system 80 shown in FIG. 8, each pair oflenses 5, 81 and 82 is disposed one on the other. Not only is theshaping aperture 83 disposed between the condenser lens 5 on thedownstream side and the irradiation lens 81 in the upstream side in theirradiation direction of the electron beam B, but the blanker 84 isdisposed between the irradiation lens 81 on the upstream side and theirradiation lens 81 on the downstream side in the irradiation directionof the electron beam B. Not only is the reticle 85 disposed between theirradiation lens 81 on the downstream side 81 and the image forming lens82 on the upstream side, but the contrast aperture 86 is disposedbetween the image forming lens 82 on the upstream side and the imageforming lens 82 on the downstream side.

Since the electron beam exposure system 90 is equipped with a Schottkyemitter 1 emitting an electron beam B high in brightness, an angle ofthe electron beam B converging to one point on the reticle 85 can besuppressed to be small, thereby enabling a spatial resolution of anexposure pattern focused on the substrate W to be improved.

(2) In the example, an AC etching method is adopted to form the Schottkyemitter 1 so as not to reveal a (100) crystal plane in the emitter sidesurface portion on the emission side outward from the suppressorelectrode 2, while no limitation is placed on an AC etching method asfar as a (100) crystal plane is not revealed in the emitter side surfaceportion.

(3) In the example, a Schottky emitter 1 has a shape where no (100)crystal plane is revealed in the emitter side surface portion on theemission side outward from the suppressor electrode 2, while theSchottky emitter 1 is not necessarily required to have a shape shown inFIG. 5B unless an unnecessary extraction current is suppressed. Forexample, as shown in FIG. 5A, unnecessary (100) crystal planes may berevealed forward from the suppressor electrode 2.

The invention may be embodied in other specific forms without departingfrom the spirit or essential attributes thereof and, accordingly,reference should be made to the appended claims, rather than to theforegoing specification, as indicating the scope of the invention.

In this specification, the verb “comprise” has its normal dictionarymeaning, to denote non-exclusive inclusion. That is, use of the word“comprise” (or any of its derivatives) to include one feature or more,does not exclude the possibility of also including further features.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

What is claimed is:
 1. An electron beam control method comprising anelectron beam generating step of emitting electrons from the conical endof an emitter sharpened in the shape of a cone on the emission side ofan electron beam by using a Schottky effect under an electric fieldapplied to the conical end to thereby generate an electron beam, themethod further comprising: a curvature radius adjusting step ofadjusting a radius of curvature of the conical end; an electron gunfocal length control step of controlling an electron gun focal length ofan electron beam by a radius of curvature adjusted in the curvatureradius adjusting step; and an angular current density control step ofcontrolling an angular current density of an electron beam with a focallength controlled by the electron gun focal length control step, whereinthe electron beam generating step is conducted each time emission of anelectron beam in a state where an angular current density is controlledafter the angular current density control step.
 2. The electron beamcontrol method according to claim 1, wherein the radius of curvature isselected in the range of 1 μm or more in the curvature radius adjustingstep.
 3. The electron beam control method according to claim 1, furthercomprising: a protrusion length adjusting step of adjusting a protrusionlength that is a length of the conical end from a suppressor electrodewhich is on the side opposite the emission side among the two electrodesestablishing an electric field and to which a negative voltage isapplied when the conical end is protruded on the emission side outwardfrom the suppressor electrode; and a combination range setting step ofsetting a combination range of the protrusion length and the radius ofcurvature based on the value of the electric field, wherein in theprotrusion length adjusting step, a protrusion length is selected in thecombination range at a radius of curvature adjusted in the curvatureradius adjusting step based on the combination range.
 4. The electronbeam control method according to claim 3, wherein the radius ofcurvature is selected in the range of 1 μm or more and 4 μm or less inthe curvature radius adjusting step, and a protrusion length is selectedin the range of 200 μm or more and 1500 μm or less from the combinationrange at the radius of curvature adjusted in the curvature radiusadjusting step based on the combination range and the radius ofcurvature.
 5. The electron beam control method according to claim 1,further comprising: an emitter forming step of adjusting the protrusionlength of the conical end and, also, applying the emitter with formingnot revealing a {100} crystal plane in the side surface portion of theemitter on the emission side outward from the suppressor electrode thatis applied with a negative voltage and located on the side opposite theemission side among the two electrodes applying the electric field. 6.An electron beam generating apparatus comprising: an emitter having theconical end sharpened in the shape of a cone on the emission side of anelectron beam; and two electrodes applying an electric filed to theconical end of the emitter, wherein the electric field is applied to theconical end to thereby emit electrons using a Schottky effect, so thatan electron beam is generated, the electron beam generating apparatushaving an improvement that a radius of curvature of the conical end is 1μm or more to control an electron gun focal length.
 7. The electron beamgenerating apparatus according to claim 6, wherein a suppressorelectrode and the emitter are disposed so that a protrusion length, thatis a length to the topmost point of the conical end from the suppressorelectrode, is in the range of 200 μm or more and 1500 μm or less whenthe conical end is protruded on the emission side outward from thesuppressor electrode, wherein the suppressor electrode is on the sideopposite the emission side among the two electrodes establishing anelectric field and carry a negative voltage, and a radius of curvatureof the conical end is in the range of 1 μm or more and 4 μm or less. 8.The electron beam generating apparatus according to claim 6, wherein theemitter has a form not to reveal a {100} crystal plane in the emitterside surface portion on the emission side outward from a suppressorelectrode, located on the side opposite the emission side of among thetwo electrodes establishing an electric field, and carry a negativevoltage.
 9. An apparatus using electron beam generating means including:an emitter having the conical end sharpened in the shape of a cone onthe emission side of an electron beam; and two electrodes applying anelectric filed to the conical end of the emitter, wherein the electricfield is applied to the conical end to thereby emit electrons using aSchottky effect, so that an electron beam is generated, the electronbeam generator having an improvement that a radius of curvature of theconical end is 1 μm or more to control an electron gun focal length, theapparatus further comprising: a processing device conducting apredetermined processing based on an electron beam generated by theelectron beam generator.
 10. The apparatus using electron beamgenerating means according to claim 9, wherein a suppressor electrodeand the emitter are disposed so that a protrusion length, that is alength to the topmost point of the conical end from the suppressorelectrode, is in the range of 200 μm or more and 1500 μm or less whenthe conical end is protruded on the emission side outward from thesuppressor electrode, wherein the suppressor electrode is on the sideopposite the emission side among the two electrodes establishing anelectric field and carry a negative voltage, and a radius of curvatureof the conical end is in the range of 1 μm or more and 4 μm or less. 11.The apparatus using electron beam generating means according to claim 9,wherein the emitter has a form not to reveal a {100} crystal plane inthe emitter side surface portion on the emission side outward from thesuppressor electrode, located on the side opposite the emission sideamong the two electrodes establishing an electric field, and carry anegative voltage.
 12. The apparatus using electron beam generating meansaccording to claim 9, wherein the apparatus is an electron probemicroanalyzer conducting an analysis or observation of a specimen, andthe processing device conducts an analysis or observation of a specimenby irradiating the specimen with an electron beam to obtain an X rayimage based on X rays generated from the specimen, or by irradiating aspecimen with an electron beam to obtain an electron beam image based onsecondary electrons or reflected electrons generated from the specimen.13. The apparatus using electron beam generating means according toclaim 9, wherein the apparatus is an X ray tube, and the processingmeans is a target generating X rays by collision with an electron beam.14. The apparatus using electron beam generating means according toclaim 9, wherein the apparatus is an electron beam lithographyapparatus, and the processing device conducts lithography using anelectron beam.
 15. An emitter generating an electron beam in which theconical end of an emitter on the emission side of an electron beam issharpened in the shape of a cone and is applied with an electric fieldto thereby emit electrons using a Schottky effect, the emitter having animprovement that a radius of curvature of the conical end is 1 μm ormore to control an electron gun focal length.
 16. The emitter accordingto claim 15, wherein a radius of curvature of the conical end is 1 μm ormore and 4 μm or less.
 17. The emitter according to claim 15, which hasa form not to reveal a {100} crystal plane in the emitter side surfaceportion.